Programmable pattern generators are well known for applying patterns to articles in an industrial process. For example, U.S. Pat. No. 4,389,971 discloses an apparatus for regulating the application of glue to a defined area of a moving paper board box blank including a sensor for detecting the presence of the leading and trailing edges of a glue application area. A pulse generator provides pulses indicative of the movement of the application area. The sensor initiates a counting action at detection of the leading edge of a glue application area such that after a predetermined number of pulses have been counted by a counter (representative of the fixed distance between the sensor and glue applicator), glue application is initiated. The sensor initiates a second counting action at detection of the trailing edge of the glue application area and the counter stops glue application after the same predetermined number of pulses representative of the sensor glue applicator fixed distance.
Accordingly, this prior art United States patent teaches the general concept of feedback control in a glue application system responsive to leading and trailing edges of an object to which the adhesive is applied. The disadvantage of this system is that the rate of application of glue is dependant upon the detected distance between flaps (i.e. length of glue application area) relative to movement of the conveyor. The result is that in the event the conveyor slows down or speeds up, initiation and termination of glue application is controlled in terms of area instead of rate of application of the adhesive. In other words, in the event the conveyor runs more slowly than usual, a larger amount of adhesive will be applied over the same area.
U.S. Pat. No 4,357,900 discloses an apparatus for automatic spray control of articles carried by a conveyor. A sensor monitors the position of the article and generates signals representative thereof. A second sensor generates additional signals to the control device when a article passes a predetermined point. The control device controls the spraying process responsive to the signals.
As with the aforementioned U.S. Pat. No 4,389,971, the system disclosed in U.S. Pat. No 4,357,900 requires a sensor for detecting motion of the conveyor and producing a clock signal to mark each time the conveyor has moved a predetermined increment along its path. Thus, this latter system suffers from the same disadvantage as the prior art system discussed above in that there is no feedback control of adhesive application responsive to the object length with reference to an absolute time frame. Instead, the latter system controls adhesive application only with respect to relative movement of the conveyor.